The technology relates to a fusing device for fixing a developer image on a sheet. The technology further relates to an electrophotographic image forming apparatus, such as a copier, a printer, or a facsimile machine, provided with such fusing device.
An electrophotographic image forming apparatus transfers a developer image (hereinafter referred to as a toner image) on a sheet, and then fuses and fixes developer (hereinafter referred to as toner) on the sheet with a fusing device. The fusing device includes a heat roller and a pressure roller.
The heat roller has a first internal heater lamp positioned along an axial direction thereof. The first heater lamp heats the heat roller from the inside, and the heat roller in turn heats a sheet being passed between the heat roller and a pressure roller, thereby fusing toner on the sheet.
The pressure roller is supported, with its axis parallel to the axial direction of the heat roller, in contact with an outer surface of the heat roller. The pressure roller pressurizes a sheet being passed between the heat roller and the pressure roller, thereby fixing fused toner onto the sheet. The pressure roller has a second internal heater lamp positioned along an axial direction thereof. The second heater lamp heats the pressure roller from the inside. The pressure roller in turn heats the surface of the heat roller and a sheet being passed between the heat roller and the pressure roller, in a supplemental manner.
Each of the heat and pressure rollers is supported on both axial ends by a frame of the image forming apparatus.
The heater lamp is required to be controlled in such a manner that the heat roller has a uniform surface temperature for uniform transfer of toner on a sheet. Also, image forming apparatus are normally supplied with power through outlets provided in offices or the like, i.e., by a commercial power supply. It is thus essential to develop an image forming apparatus operable at 100V/15 A, which is a common value for commercial power.
Meanwhile, many multifunctional image forming apparatus have been recently developed that are provided with not only a printing device but also optional devices such as an automatic document reader (i.e., a scanner), a postprocessing device (with postprocessing functions such as of stapling or punching), or a Large-Capacity Cassette (LCC).
Commercial power is insufficient for such multifunctional image forming apparatus, resulting in power shortages in some sections of the apparatus. To the sections having power shortages, power originally intended for consumption by a fusing device is diverted. This causes a decrease in power supply to the fusing device, thereby preventing an outer surface of the heat roller from being held at a constant temperature and therefore causing degradation in fusing performance.
In a case where total power supplied by the commercial power supply is 1500 W, for example, power available to the apparatus is approximately 1200 W to 1300 W in view of power fluctuations and safety standards. Of the available power, 200 W to 300 W of power is allocated to activate and control the apparatus itself, and 800 W to 1000 W of power is allocated to heaters in the fusing device. When optional devices such as described above are installed, 200 W to 400 W of power is subtracted from the power allocated to the heaters, to be allocated to activate and control the optional devices.
The temperature of the heat roller is high in the center area and decreases towards each of its axial ends, even when the entire heat roller is uniformly heated. This is because the heat escapes from each axial end to the frame of the image forming apparatus through a rotation shaft and shaft bearings. Thus, the heat roller has a plurality of internal heater lamps each having a heating element, and the heating elements are arranged at different positions along the axis. The arrangement of the heating elements allows a greater amount of heat to be generated at each axial end of the heat roller than in the center area, thereby ensuring that the heat roller maintains a substantially uniform distribution of surface temperature across its axial direction.
Recently, fusing devices have been developed that use an external heat roller as a supplementary heating member for heating an outer surface of a main heat roller quickly. Such fusing devices are disclosed by JP H10-149044A and JP 2005-221712A. The external heat roller is supported with its axis parallel to an axial direction of the main heat roller, and has internal heater lamps arranged along the axis. Generally, the external heat roller is formed so as to be at least equal in axial length to the main heat roller so that the entire surface of the main heat roller is heated.
The heat control as described earlier, however, involves complicated control of maintaining a substantially uniform distribution of surface temperature of the heat roller across its axial direction within a limited amount of available power. This is particularly the case with fusing devices provided with an external heat roller because such devices involve more complicated control of energizing heater lamps provided in main and supplementary heat rollers in an appropriate way.
A feature is to provide a fusing device that enables simplified heating control for maintaining a uniform distribution of surface temperature of a heat roller, and an image forming apparatus provided with such fusing device.